1. Field of the Invention
The present invention relates generally to the field of wire bonding. More particularly, the present invention relates to a ball-wedge bonding process than is capable of preventing polymer such as polyimide cracking.
2. Description of the Prior Art
As known in the art, wire bonding is a technology used to attach a fine wire, usually 1 to 3 mils in diameter, from one connection pad to another, completing the electrical connection in an electronic device. The pads can be bond sites on the semiconductor chip or metallized bond sites on interconnection substrates. Semiconductor die can also be wire bonded to metal lead frames as is done in plastic encapsulated devices.
The methods presently used to wire bond include thermocompression, ultrasonic and thermosonic. Other types of bonding technology such C4 (Controlled Collapsed Chip Connection or Flip Chip) or TAB (Tape Automated Bonding) also have been used in parallel with conventional wire bonding technology. Wire bonding continues to be popular and dominant in the field of bonding technologies in the industry.
In the past, a large proportion of all semiconductor-device field failures were caused by wire bonds and the number of known failure mechanism were quite limited. Through the improvement of bonding technology the reliability of wire bonds is increasing, as is our understanding of the failure modes, though they continue to plague new manufacturing lines.
There are basically two forms of wire bonds. They are wedge bonds and ball bonds. Recent study shows that about 90% of all electronic packages and assemblies are produced using ball bonds and about 10% are produced with wedge bonds. An increase in the use of ball bonding is expected as semiconductor devices increase in functionality and decrease in size causing smaller bond pads and closer bond pad spacing.
The most widely used wire materials are gold (Au) and aluminum (Al), however, silver (Ag) and copper (Cu) are also used. Copper wire (ball bonding) has gained considerable attention due to its economic advantage and strong resistance to sweeping (leaning of the stress relief loop until it touches an adjacent bond wire). Bonding these wire materials to different pad materials creates different metallurgical systems called intermetallics. Gold was the original material used when wire-bonding technology was developing. Aluminum has become popular due to its good electrical performance and lower cost. Gold wire can be bonded in the shape of a wedge or a ball. Ball bonds can be used in very tight spacing.
One method of wire bonding is thermosonic or ball-wedge bonding. The thermosonic bonding is used for Au wires and currently comprises about 90% of all wire bonding. Typically, it is done at temperatures of around 100° C. to 240° C. Bonding is formed when the ultrasonic energy combines with the capillary technique of thermocompression bonding. Occasionally, it is used for Au wedge-wedge bonding, but mostly it is best suited for a ball-wedge bond.
The machine used to make ball-wedge bonds incorporates a “flame-off” arm. At the end of each full bonding cycle an arc is struck between the arm and the end of the wire, producing a ball. The ball is deformed as it is welded to the IC (integrated circuit), the second bond (known as the wedge, capillary-wedge, termination bond or stitch bond) is then made to the substrate.
The prior art ball-wedge bonding technology has a drawback in that during wedge boning on the Au pad of an IC die, stress is exerted on the Au pad and the underlying materials layers, inducing undesirable crack defects in the IC die.